The present invention relates to an alloy for a heat transfer pipe, particularly to an alloy for a heat transfer pipe on the secondary side of a nuclear reactor.
Presently, a heat transfer pipe of a steam generator in a nuclear reactor such as a pressurized water reactor is made of an only annealed alloy or Alloy 600 (trade name, 75%Ni-15%Cr-8%Fe) which has further been subjected to a specific thermal treatment (700.degree. C..times.15 hr). However, recent researches have revealed that Alloy 600 which is the alloy for the heat transfer pipe has the following problems. That is to say, a stress corrosion cracking sometimes occurs owing to an alkaline concentrate in the gap between the heat transfer pipe and a pipe-supporting plate in environments (alkaline environments including ammonia and hydrazine and having a pH of 9.2 to 9.5 and a temperature of 280.degree. C.) on the secondary side of the nuclear reactor, and a pitting corrosion tends to take place owing to Cl.sup.- ions in leaked seawater in the same environments on the secondary side of the nuclear reactor. Further, this pitting corrosion will deeply penetrate and the number of the pitting corrosions will augment with the increase in a concentration of the Cl.sup.- ions.
Heretofore, it is known that the specific thermal treatment is given to a nickel based alloy for the sake of the improvement in a stress corrosion cracking resistance (hereinafter referred to as the SCC resistance). For example, Japanese Patent Disclosure No. 25216/1979 discloses a method in which after a final annealing treatment, the alloy is successively heated and retained at a temperature of 550.degree. to 850.degree. C. for a period of 1 to 100 hours in order to deposit a carbide on crystal boundaries, thereby heightening the SCC resistance. Even by such a conventional technique, however, a resistance to the stress corrosion cracking caused by the alkaline concentrate, i.e. an alkali stress corrosion cracking resistance and the pitting corrosion resistance cannot be improved. Further, in fact, the nickel based alloy obtained by such a conventional method cannot always provide a satisfactory combination of the alkali stress corrosion cracking resistance and the pitting corrosion resistance.